The Lao border town 2 km before you cross into Vietnam at LaoBao. It's a typical border town, and fully functional. You can eat, drink, change money (dollars and kip into Vietnamese Dong at reasonable rates), buy girls and stay overnight. Apart from that, it's a dusty hole. Getting out of Laos is, as is usual for Laos, laid back and hassle free. Time your run from Xepon so you can cross over into Vietnam and make the 20 km up to KheSan township (see Leg 3) for the night. It'll take about 3 hours (max.) from here, including getting through Vietnamese customs.
Vietnamese customs, by the way, are very good (read: slack) these days. Crossing at LaoBao used to be a nightmare. They would keep you waiting and search everything from your toothpaste to your tape/book collection, the latter which they would invariably keep.